Fix and re-enable TextViewActivityMouseTest

The main problem was in MouseUiController: when generating an ACTION_UP
event, button state was not being reset so the event was getting
filtered out by the check in Editor.shouldFilterOutTouchEvent.

This CL also removes the slow negative assertions for the selection
handles (assertNoSelectionHandles).

There is one remaining issue in testDragAndDrop_longClick() which for
now is still suppressed. The real for this will be in a separate CL
because it requires updates to the touch state logic of Editor.

Bug: 29591177
Test: Ran tests
  atest FrameworksCoreTests:TextViewActivityMouseTest
  atest FrameworksCoreTests:TextViewActivityTest
Change-Id: I6ce73bf30d9c49ec1b5a5930db0f9cf52929bf43